Given that ImageJ is constantly evolving, sometimes an old plugin may only run with a previous version of ImageJ and not the newest. Where can one get those older ij.jar files?

The previous versions of ImageJ (i.e. the jar files) are available at:

http://rsb.info.nih.gov/ij/download/jars/

Just copy the i.jar file in the ImageJ folder and restart the program.

For the latest version see this FAQ.